Lipoxygenases are a class of enzymes that oxidize unsaturated fatty acids to fatty acid hydroperoxides, incorporating molecular oxygen. These products are then transformed into a variety of molecules with important biological activities. Although these enzymes have been the subject of a great deal of study, the mechanism and kinetics of this oxidation are not well understood. This proposal describes measurements of the enzyme kinetics designed to illuminate the kinetic mechanism of this enzyme and some of its unusual features, by using freeze-quench EPR spectroscopy and specifically deuterated substrates. The results of these experiments will lead to a greater understanding of the rate limiting step of the mechanism, the nature of important intermediates, and the origins of the large isotope effect.